1. Field of the Invention
The present invention relates to atomic layer deposition coatings onto porous materials to provide nuclear fuel pellets and claddings of superior mechanical and thermal properties, which will help increase such components' operational life and improve the efficiency and safety of current light water nuclear reactors.
2. Background of the Invention
The inside of a nuclear reactor is an extremely harsh environment. Not only are materials subject to temperatures as high as 1800° C. at the center of the fuel pellet and to highly corrosive steam, but they are also subject to damage from neutron bombardment.
Neutrons cause fission reactions in typical Light Water Reactors (LWRs). LWRs utilize the energy produced from fission reactions occurring in the fuel elements to heat water or steam in the reactor core. This water or steam travels through a heat exchanger to heat clean water into clean steam, and this clean steam turns downstream turbines to produce mechanical energy or motion. The mechanical energy turns a generator which results in the production of electricity.
The water or steam in the reactor core is part of a closed loop that does not mix with, or contaminate, the clean water used to turn the turbines. A representation of a typical LWR arrangement is depicted in FIG. 1A. A nuclear reactor core 20 contains a series of co-planarly arranged fuel rods 24 between which are positioned control rods 22. The control rods 22 are made of highly neutron absorbent materials such as silver, indium, hafnium, boron, and cadmium. Depending on power requirements called for by the grid, the control rods 22 are either partially or fully inserted or removed from between the fuel rods to moderate the flux of neutrons, and therefore the amount of fission. This moderation is proportional to the amount of energy produced.
As depicted in FIG. 1B, the fuel rods 24 house the fissile material, typically in the form of fuel pellets 26. In a typical LWR, the fuel pellets 26 contain the fissile material, usually in the form of a sintered oxide, such as uranium dioxide. Encapsulating the fuel pellets 26 is a cladding layer 28, which is typically made of zirconium or a zirconium alloy.
As can be seen in FIG. 1C, the cladding material 28 surrounds the fuel pellet 26, but a gap 30 must be provided to allow for the expansion of the fuel pellet 26 and the cladding layer 28. Expansion occurs primarily because of the nuclear irradiation. Because the fuel pellet 26 and cladding 28 are constantly being bombarded by neutrons, individual atoms on the lattice structure of the fuel pellet 26 and cladding 28 are displaced. During the operational life of the fuel pellet 26 and cladding 28, each atom is displaced from its lattice site thousands of times on average. These displacements lead to the agglomeration of defects, which can create large voids in the lattice structure. The structural changes to the atomic lattice happen randomly in prior art sintered fuel pellets and in zirconium-based claddings, which means that the fuel pellets 26 and cladding 28 do not expand uniformly or quickly. Structural changes to the atomic lattice do not reach dynamic equilibrium before the fuel pellets 26 have materially degraded and need to be replaced. Further, the cladding 28 can no longer be trusted to contain the nuclear fuel.
The mechanical degradation of the fuel pellet 26 and cladding 28 raises concerns of contamination. Maintaining the separation of reactor core coolant water and clean water for powering the turbines is critical to the operation of a nuclear power plant. Contamination of the clean water can happen through a variety of circumstances. Meltdown, in particular, can lead to a severe breach of containment. Meltdown occurs when a component or components of the reactor core melt, releasing radioactive materials, including the fuel and fission products. When the fuel pellets 26 and cladding 28 are mechanically vulnerable, the possibility of leaching nuclear material into the coolant water is greatly increased.
Typical fuel pellets are made of sintered uranium dioxide (UO2). The uranium present in the pellets is mostly uranium-238, which has been enriched to contain approximately three percent uranium-235. The uranium-235 is the major fuel of the LWR, but the uranium-238 is fissionable and produces plutonium-239, which also fuels the LWR. In some reactors, the pellets are made of both uranium and plutonium oxides and are referred to as mixed oxide fuels.
Claddings 28 are commonly made of a zirconium-based alloy. Zirconium has exceptional properties for use in nuclear reactors including low-neutron absorption, high hardness, ductility, and corrosion resistance. Zirconium alloys typically contain greater than 95% zirconium and other metals, such as tin, niobium, iron, chromium, and nickel. However, zirconium is prone to hydrogen embrittlement at high-temperatures, especially after a loss-of-coolant-accident (LOCA). The zirconium will react with the water or steam and form an oxide, which produces hydrogen gas. Not only does the presence of hydrogen gas increase the risk of an explosion, but it causes hydrogen embrittlement. The hydrogen embrittlement leads to blistering and cracking of the cladding 28 through which radioactive materials can escape. Further, despite zirconium's low neutron absorption, the cladding 28 experiences significant radiation expansion during its operational life.
In a nuclear reactor, the fuel capacity of the uranium dioxide is generally not completely consumed. Over the 3-5 year operational life of a prior art uranium dioxide pellet, only approximately 5% of the available fuel has been fissioned. Replacement of the fuel pellets 26 and cladding 28 is necessitated by the mechanical degradation of those components. Therefore, not only is much of the fuel wasted, but it also must be carefully stored. Storage can be accomplished on-site, but often times the radioactive waste materials must be moved to other locations to accommodate the large amount of storage necessary.
Another problem facing prior art fuel pellets 26 and claddings 28 is the heat conduction from the center of the pellet 26 to the exterior of the cladding 28. Thermal transport of heat from the fuel elements is critical for optimized reactor operations. The performance of nuclear fuels strongly depends on the operating temperature. Optimized thermal transport also extends the operation limit of nuclear fuels. Fuel porosity and fuel stoichiometry are critical factors in thermal transport.
The heat conduction of uranium dioxide is poor relative to that of the cladding material. This can cause high heat build-up within a fuel rod, leading to failure. Over time, temperatures at the center of the fuel pellet 26 become much higher than at the exterior of the cladding 28. At an operational temperature of 1800° C., the heat conduction of uranium dioxide is approximately 2 W/mK (where mK is meters-Kelvin), while the heat conduction of zirconium is 35 W/mK. The poor heat conduction is exacerbated by the expansion gap 30 between the pellet 26 and the cladding 28.
A need exists in the art to increase the efficiency and operational life of nuclear fuel pellets. Such a pellet would allow for a more complete fission of the fuel material. More fissions would increase the operational life of the fuel pellet, which would cut down on the amount of pellets that would have to be used. This, in turn, would reduce the waste produced and lessen the need for storage facilities and transportation to storage facilities.
Another need exists in the art for a cladding that is able to withstand extreme temperatures, oxidation, and hydrogen embrittlement. The cladding should resist bubbling and cracking and prevent the escape of radioactive materials. Further, such a cladding should have a long operational life.
Still another need exists in the art for a fuel pellet and cladding that is designed to aid in the prevention of meltdown. Such a pellet would allow for the efficient conduction of heat out of the nuclear reactor core, preventing the build-up of heat and an increase in temperature. This improved conductivity would also allow for a more efficient transfer of energy to the water in the heat exchanger, which would mean that more useful energy is produced by each fission.